While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by C. Tang et al. (J. Applied Physics, Vol. 65, 3610 (1989)). The light-emitting layer commonly consists of a host material doped with a guest material, otherwise known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron-transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
EL devices in recent years have expanded to include not only single color emitting devices, such as red, green and blue, but also white-devices, devices that emit white light. Efficient white light producing OLED devices are highly desirable in the industry and are considered as a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
Since the early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. Nos. 5,061,569, 5,409,783, 5,554,450, 5,593,788, 5,683,823, 5,908,581, 5,928,802, 6,020,078, and 6,208,077, amongst others.
Notwithstanding all of these developments, there are continuing needs for organic EL device components, such as hosts for light-emitting layers, light-emitting materials, materials for hole transporting layers and/or hole injecting materials, that will provide even lower device drive voltages and hence lower power consumption, while maintaining high luminance efficiencies.
The use of acridone materials in OLED devices are known. JP 8-67873 and JP2001-173772 describe the use of N-substituted acridones in light-emitting layers.
JP2000-056408, JP2005-089544, JP2001-244076 and U.S. Pat. No. 5,811,834 all disclose the use of bis-acridones in light-emitting layers.
U.S. Pat. No. 6,894,307 describes the use of acridones as intersystem crossing agents in light emitting layers.
CN1660844 discloses quinacridones linked to a carbazole group with saturated alkyl chains. JP2005093098 and JP2007191439 disclose quinacridones linked to carbazole groups but not through the nitrogen of the quinacridone.
However, these devices do not have all desired EL characteristics in terms of high efficiency and low drive voltages.
Notwithstanding all these developments, there remains a need to increase efficiency and lower drive voltage of OLED devices, as well as to provide embodiments with other improved features.